The genetic basis of antibiotic resistance in Bacteroides has been investigated. The B. ochraceus drug resistance plasmid pGD10 has been shown to belong to the FII incompatibility group and is closely related to the R plasmids RI, R100, and R6. Clindamycin resistance in clinical isolates of Bacteroides can involve at least two different mechanisms, but neither mechanism is mediated by transmissible plasmids. The process of DNA transformation has been investigated in Bacteroides. Bacteroides do not appear to transform with either chromosomal or plasmid DNA. The mechanism of the bactericidal action of metronidazole on Bacteroides has been studied by extracting plasmid DNA from metronidazole-treated cells. Killing by metronidazole does not involve DNA breakdown as was reported by other investigators.